The present invention relates to an adaptive array antenna and a method for calculating a calibration amount of a receiving system of the adaptive array antenna and a method for calibration.
Generally, the adaptive array antenna is used for beam control of array antennas. There are two kinds of algorithms for the beam control, which are an interference suppression tracking type and a maximum gain tracking type. In the interference suppression tracking type, tracking is performed having a null point to interference waves and having strong directivity to desired waves. In the maximum gain tracking type, tracking is performed such that the receiving level of the antenna is maximized. In each of the types, a mobile station can be tracked by a main beam. At the current time, the spacing between elements of the array antenna is usually xcex/2 as shown in FIG. 12. The reason for this is that a grating lobe may occur when the spacing is more than xcex/2 as shown in FIG. 13. The grating lobe may increase interference since the main lobe can be distracted to an unnecessary direction. Although the width of the beam narrows, gain increase by this is not obtained.
Because the spacing between elements of the array antenna shown in FIG. 12 is narrow, the correlation between the elements is very high. Therefore, when the receiving level deteriorates due to fading, the deterioration influences all elements 1-8 which are included in the array antenna so that the deterioration can not be compensated for. Especially, the phenomenon is remarkable for a small sized array antenna which has about 4 elements. In addition, in the interference suppression tracking type algorithm, when there are interference waves coming from near the direction of desired waves, the interference suppression capability deteriorates remarkably since the interference waves exist in the main lobe.
That is, in the narrow element spacing adaptive signal processing, correlation of envelope and signal between elements is extremely strong and the phase deviation is less than a wave length. In the interference suppression tracking type algorithm, wait (phase and amplitude) of each antenna element is obtained such that the interference waves cancel each other out and the desired waves do not cancel each other out. Interference waves which come from a direction different enough from the desired waves are input into the antenna as a signal having strong envelope and signal correlation like the desired waves. However, since the arriving angle is different, the phase difference of the interference waves between elements is different from the phase difference of the desired waves. As a result, the desired waves are not necessarily in opposite phase for a wait in which the interference waves are in opposite phase. In many cases, the desired waves operate as in phase. On the other hand, when the arriving direction of the interference waves is close to the desired waves, the amount of phase shift of the desired waves and the interference waves is almost the same. Therefore, the attempt to cancel the interference waves may result in canceling the desired waves so that the interference suppression capability deteriorates.
On the other hand, since a diversity antenna is designed such that the correlation between elements becomes small, the spacing between elements 21-28 is large as shown in FIG. 14. Since the correlation is small, when the receiving level of an element declines, the receiving level of another element may be high. Generally, a maximal ratio combining (MRC) algorithm is applied. According to the maximal ratio combining algorithm, receiving waves of each of the antenna elements 21-28 are synthesized after assigning weights of envelop level of the receiving waves after placing the receiving waves in phase. According to this algorithm, the concept of beam control is not applied because the spacing between elements is large, thus, many ripples exists in the envelope which is the beam of each element. Therefore, tracking is not performed since too many main beams exists. Thus, the gain increase by narrowing the beam can not be expected. According to the algorithm, when there are the interference waves, the influence is directly exerted. Because, as mentioned above, in the synthesizing method, amplitude and phase are controlled such that signals of all elements can be received at maximum gain, and the interference waves and the desired waves are treated without distinction. Accordingly, the method of the maximum ratio synthesizing diversity shown in FIG. 14 is effective for improving receiving characteristics of a desired station that has deterioration due to fading. However, the method does not contribute to improved interference characteristics.
As mentioned above, the narrow element spacing adaptive array antenna of the interference suppression tracking type is effective in suppressing interference waves other than from main beam. However, the antenna has no effect for suppressing interference waves in the main beam and fading. On the other hand, although the diversity antenna which has the wide element spacing can compensate for deterioration of characteristics of the desired waves due to fading, the diversity antenna has no effect pertaining to interference waves.
In addition, there are two more combinations of antenna placements (narrow element spacing, wide element spacing) and algorithms (maximum ration synthesizing, interference suppression). First, the combination is the maximum gain tracking type which uses the narrow element spacing as shown in FIG. 15 and the maximum ratio synthesizing algorithm. Second, the combination is the interference suppression type which uses the wide element spacing as shown in FIG. 16. In the method shown in FIG. 16, the antenna is set for diversity configuration and the algorithm is the interference suppression type. According to the method, capability of interference wave suppression is kept as the basic characteristic of the algorithm. In addition, fading can be compensated for since the correlation between elements is small in the diversity configuration. Especially, the characteristic is remarkable when the angle of spreading of arriving waves is large. A wait (phase and amplitude) can be determined such that many coming element waves of the interference waves are statistically canceled out because phase differences due to the arriving angle are different. Therefore, even if the arriving angles are the same, a wait can be generated such that the desired waves become inphase and the interference waves become opposite phase.
However, according to the combination method of the maximum gain tracking type which uses the narrow element spacing shown in FIG. 15 and the maximum ratio synthesizing algorithm, high gain can be obtained and the desired waves can be tracked with an antenna similar to the adaptive array antenna shown in FIG. 12. However, the method has no effect on interference waves and fading. In addition, according to the combination method using the wide element spacing and the interference suppression type shown in FIG. 16, gain increase can not be obtained because the wideness of the element spacing prevents tracking by the main beam.
One of the objects of the present invention is to solve the above-mentioned problems. The object is to provide an adaptive array antenna which has diversity effects such as fading compensation or the like, eliminates the interference waves from the same direction and increases gain by main beam tracking.
In order to obtain effects which are diversity effects such as fading compensation or the like, removing interference waves from the same direction and increasing gain by main beam tracking, the adaptive array antenna needs to be accurately calibrated. In the following, the calibration will be described.
In the adaptive array antenna, it is necessary that amplitude ratio and phase difference in a high frequency band occurring between element antennas are maintained to baseband on which signal processing is performed. Generally, since a cable, an amplifier, a filter, a mixer, a converter and the like have different electronic characteristics, it is difficult to equate the amplitudes and phases of all the elements. (In the following, the electronic difference between elements will be called xe2x80x9cindividual variationxe2x80x9d.) In addition, it is practically impossible to equate the amplitudes and phases since there are differences due to temperature in addition to the general differences. Therefore, as shown in FIG. 17, it is conceivable to measure the amplitude ratio and the phase difference between the elements by providing the same calibration signals to each antenna and correct the amplitude ratio and the phase difference beforehand based on the measurement in order to keep the amplitude ratio and the phase difference constant within a fixed period.
The calibration signal can be realized by inserting the signal into a frame format at each channel during constant time intervals of one minute or ten minutes or the like. A calibration signal input terminal may be described as a switch type shown in FIG. 18A in the following description. In addition, the terminal may be a type that connects to an antenna cable or the like electromagnetically as shown in FIG. 18B. In the switch type, communication is interrupted during the switching. On the other hand, the type using an electromagnet connection has the effect that there is no such interruption. In the FIGS. 18A, 18B, the array antenna is connected to the terminals a and b and the calibration signal is applied to the terminal c.
The part where the calibration signal is applied is called a calibration signal coupling part, which includes the calibration signal input terminals of the switch type and the electromagnetically connecting type.
FIG. 17 shows the array antenna which includes antenna elements #1-#4. Signals received by each of the antenna elements are applied to a distribution and synthesizing part 134 via filters 103-106 and high frequency amplifiers 107-110. In the distribution and synthesizing part 134, the signal received by the antenna is distributed to channels. Therefore, the signals after the distribution and synthesizing part 134 are transmitted to a plurality of channels. However, one channel in the plurality of channels is shown in FIG. 17. The received signals distributed by the distribution and synthesizing part 134 are added at a signal adder 132 via mixers 111-114, filters 115-118, intermediate frequency amplifiers 119-122, A/D converters (analog digital converters) 123-126 and waits 128-131. An adaptive signal processing device 133 controls amplitude and phase of the waits 128-131. As a result, the received signals are transmitted to a base station signal processing circuit.
The output from a calibration signal generator 101 is split in four by a signal splitter 102 and at the same time the calibration signals are applied to filters 103-106 via cables 175-178 and calibration signal input terminals 166-169 in the antenna elements #1-#4. These signals are transmitted to the base station signal processing circuit in the same way as received signals. At the time, output signals from the A/D converters 123-126 are applied to a calibration amount calculator 127. The calibration amount calculator 127 compares the amplitude and the phase of each A/D converters 123-126 with each other and calculates calibration amount for equalizing amplitude change and phase change between the antenna elements #1-#4 and the signal adder 132 in the receiving systems. The receiving system here is a system which includes a series of circuits for receiving connected to the output of the antenna. That is, the receiving system includes the filter, the high frequency amplifier, the mixer, the filter, the intermediate frequency amplifier and the AD converter. Four receiving systems are included in FIG. 17. The calibration amount is transmitted to the adaptive signal processing device 133. The adaptive signal processing device 133 stores the calibration amount in a calibration table (which is not shown in the figure). When the adaptive signal processing device 133 performs adaptive signal processing, the adaptive signal processing device 133 controls the waits 128-131 by subtracting the calibration amount.
However, the calibration signals, provided to antennas, which are regarded to be the same, have the individual variations. In FIG. 17, the calibration signal generator 101 needs to divide the output signal into the same number of signals as there are elements of the array antenna and needs to transmit the calibration signals to the calibration signal coupling part via the cables 175-178. Since the cables 175-178 and the calibration terminals have individual variations (cable characteristics and cable lengths and the like), phase differences occur in the calibration signals. As a result, there is a problem in that a calibration error occurs.
Thus, the second object of the present invention is to realize reliable calibration by eliminating effects based on the individual variations to the calibration signal.
The present invention has the following means as means for achieving the first object.
In a first embodiment of the invention an adaptive array antenna characterized in that the adaptive array antenna comprises a plurality of array antennas including a plurality of antenna elements which are spaced at intervals at which a high correlation is exhibited; where the array antennas are spaced at intervals at which the correlation is negligible; and outputs of the antenna elements being converted into baseband and adaptive signal processing being performed on the antenna elements simultaneously.
According to the first embodiment, the spacing between array antennas is a distance in which correlation can be neglected and adaptive signal processing is performed on all outputs from the antenna elements. Therefore, the adaptive array antenna has diversity effect such as fading compensation or the like, removes interference waves from the same direction and improves gain for main beam tracking.
In a second embodiment an adaptive array antenna characterized in that the adaptive array antenna comprises a plurality of array antennas including a plurality of antenna elements which are spaced at intervals at which a high correlation is exhibited; the array antennas being spaced at intervals at which the correlation is negligible; each of the array antennas performing adaptive signal processing independently; each output of the array antennas processed by adaptive signal processing being further processed by adaptive signal processing.
According to the second embodiment, an adaptive signal processing is performed in each array antenna which has a plurality of antenna elements which are separated from each other at a distance which induces a high correlation. Therefore, gain can be further improved by main beam tracking. Additionally, adaptive signal processing is further performed on each output of the array antenna on which output adaptive signal processing has been performed independently. Therefore, fading compensation can be performed more effectively.
A third embodiment is an adaptive array antenna characterized in that the adaptive array antenna comprises a plurality of array antennas including a plurality of antenna elements which are spaced at intervals at which a large correlation is exhibited; the array antennas being spaced at intervals at which the correlation is negligible; at least an array antenna of the array antennas performing adaptive signal processing; array antennas which do not perform adaptive signal processing referring to a result of the adaptive signal processing of other array antennas and adjusting phase and level of outputs of antenna elements of the array antennas which do not perform adaptive signal processing.
Further, the present invention has the following means as means for achieving the second object.
A fourth embodiment of the invention is an adaptive array antenna characterized in that the adaptive array antenna comprises: an array antenna having a plurality of antenna elements; a multi-beam synthesizing circuit for synthesizing multiple beams; a calibration signal coupling part, provided between said multi-beam synthesizing circuit and said antenna element, for inputting a calibration signal; a calibration signal generator; a calibration amount calculator; where the calibration signal generator applies a calibration signal output to the calibration signal coupling part, the calibration amount calculator calculating a calibration amount of each of receiving systems from baseband signals of the receiving systems connected to the outputs of the multi-beam synthesizing circuit and performing calibration of the receiving systems.
According to the fourth embodiment, a calibration signal is applied to the calibration signal coupling part which is provided between the multi-beam synthesizing circuit and an antenna element. And, the calibration amount is calculated for each receiving system from a baseband signal of the receiving system which is connected to the output of the multi-beam synthesizing circuit and calibration is performed on the receiving systems. Accordingly, individual variations between the calibration signals are eliminated such that reliable calibration can be realized.
A fifth embodiment of the invention is an adaptive array antenna characterized in that said adaptive array antenna comprises: an array antenna having a plurality of antenna elements; a multi-beam synthesizing circuit for synthesizing multiple beams; a calibration signal coupling part, provided between said multi-beam synthesizing circuit and said antenna elements, for inputting a calibration signal; a calibration signal generator; a calibration amount calculator; wherein the calibration signal generator applies a calibration signal output to a plurality of the calibration signal coupling parts successively, the calibration amount calculator calculating a calibration amount for each of receiving systems from baseband signals of the receiving systems connected to the outputs of the multi-beam synthesizing circuit every time the calibration signal output is applied to the calibration signal coupling part, and calibration to the receiving systems being performed by using a mean value of calculated calibration amounts.
According to the third embodiment, the array antenna which does not perform adaptive array processing refers to the result of adaptive signal processing of other array antenna, and adjusts phase and level of outputs of antenna elements of the array antenna. Therefore, the total amount of calculation can be decreased.
An aspect of the invention is the adaptive array antenna as described in one of the first three embodiments, where the adaptive signal processing is an interference suppression tracking type or a maximum gain tracking type.
This aspect of the invention defines details of the adaptive signal processing.
Another aspect of the adaptive array antenna as described in one of the first three embodiments, so that signals to which weights are assigned by the adaptive signal processing are synthesized before detection or after detection.
According to another aspect of the invention an appropriate method can be selected between synthesizing before detection and synthesizing after detection according to a communication method.
According to the fifth embodiment, a calibration amount calculation of the receiving system is performed a plurality of times and the mean value is used as the calibration amount of the receiving system. Therefore, more reliable calibration can be realized.
Another aspect of the invention is the adaptive array antenna as described in one of the fourth or fifth embodiments, where an FFT processing circuit is provided for performing calculation of multi-beam resolution within a base station in the outside of the receiving systems of the array antenna.
According to this aspect of the invention, since the FFT processing circuit is provided for performing multi-beam resolution calculations within the base station, calibration and adaptive signal processing can be performed for each antenna element.
A sixth embodiment of the invention is a calibration amount calculation method in a receiving system of an array antenna having a plurality of antenna elements, the calibration amount calculation method characterized by: applying a calibration signal generated by a calibration signal generator to a calibration signal coupling part provided in one antenna element; sending the calibration signal to a plurality of the receiving systems by a multi-beam synthesizing circuit; and calculating a calibration amount of each of the receiving systems from baseband signals obtained by detecting calibration signals of the receiving systems.
According to the sixth embodiment, individual variation between calibration signals are eliminated and reliable calibration can be performed.
A seventh embodiment of the invention is a calibration amount calculation method in a receiving system of an array antenna having a plurality of antenna elements, the calibration amount calculation method characterized by: applying a calibration signal to calibration signal coupling parts provided in a plurality of antenna elements successively; sending the calibration signal to a plurality of the receiving systems by a multi-beam synthesizing circuit provided in an array antenna simultaneously; calculating, by a calibration amount calculator connected to a plurality of the receiving systems, calibration amounts of the receiving systems from baseband signals obtained by detecting calibration signals of the receiving systems; using a mean value of the calibration amounts as a calibration amount of the receiving system.
According to the seventh embodiment, a calibration amount calculation of the receiving system is performed a plurality of times and the mean value is used as the calibration amount of the receiving system. Therefore, more reliable calibration can be realized.
An aspect of the invention is the calibration amount calculation method of the receiving systems of the adaptive array antenna as described in the sixth or seventh embodiments, where verification of calibration amount calculation is available by providing, in the outside of the receiving systems of the array antenna, an FFT processing circuit for performing calculation of multi-beam resolution within a base station.
According to this aspect of the invention, since the FFT processing circuit is provided for performing multi-beam resolution calculations within the base station, calibration and adaptive signal processing can be performed for each antenna element. In addition, calibration amount calculation can be verified.
Another aspect of the invention is a calibration method for performing calibration of a receiving system of an array antenna by performing adaptive signal processing, the processing after subtracting said calibration amount calculated by the method described in the sixth or seventh embodiments as an adaptive signal processing amount when performing adaptive signal processing for an adaptive array antenna.
According to this aspect of the invention, a calibration can be performed within adaptive signal processing without using waits for calibration.